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Armadillidium vulgare may reach a length of 18 millimetres (0.71 in), and is capable of rolling into a ball when disturbed; this ability, along with its general appearance, gives it the name pill-bug and also creates the potential for confusion with pill millipedes such as Glomeris marginata.[3] It can be distinguished from Armadillidium nasatum and Armadillidium depressum, the only other British species in the genus, by the gap that A. nasatum and A. depressum leave when rolling into a ball; A. vulgare does not leave such a gap.[4]

Armadillidium vulgare is able to withstand drier conditions than many other woodlouse species, and is restricted to calcareous soils or coastal areas.[3] It feeds chiefly on decaying plant matter, but also grazes lichens and algae from tree bark and walls.[5]

It is able to regulate its temperature through its behaviour, preferring bright sunshine when temperatures are low, but remaining in shadow when temperatures are high; temperatures below −2 °C (28 °F) or above 36 °C (97 °F) are lethal to it.[6]A. vulgare is less susceptible to cold during the night, and may enter a state of dormancy during the winter in order to survive temperatures that would otherwise be lethal.[6]

A. vulgare has also been introduced to many locations in North America, where it may reach population densities of up to 10,000 individuals per square metre.[8] It is now one of the most abundant invertebrate species in California coastal grassland habitats.[9] It has also been introduced, to a lesser extent, to sites across the world.[2]

Because of their unusual yet non-threatening appearance, some Armadillidium vulgare are kept as pets in areas throughout the U.S., typically among children. Among adults, they are often seen as unwanted (but essentially harmless) home pests.[10] Keeping a pet pill bug requires a very moist habitat with limited light and lots of decaying plant matter.[11] They can often live up to three years.[10]